1. Field of the Invention
The present invention relates to a semiconductor commutator and, more particularly, to a semiconductor commutator which is constructed by joining a semiconductor region of the first conductivity type and a semiconductor region of the second conductivity type.
2. Related Background Art
In a PN junction diode, improvement in switching speed is strongly demanded. However, in a PN junction diode using a monocrystal, since a carrier lifetime in bulk is long, when switching from the ON state to the OFF state, excessive minority carriers remain, so that the delay time for switching becomes long and there is a limitation in improvement of the switching speed.
Hitherto, in order to improve the switching speed, heavy metal, for instance, gold is used as a recombination center for a PN diode or a PN junction diode is formed on a polycrystalline semiconductor layer and a grain boundary defect is used as a recombination center, thereby reducing the lifetime of the minority carriers and decreasing the delay time.
However, in the above conventional example, in the case of using gold as a recombination center, there is a problem such that field concentration occurs in gold atoms. On the other hand, in the case of using a grain boundary defect as a recombination center, there is a problem such that since impurities are segregated in the grain boundary near a high concentration layer, field concentration occurs in the grain boundary which crosses the junction surface.
Thus, electric field breakdown occurs and a dark current component increases from a grain boundary within a depletion layer as a generation center. That is, since a particle size of polycrystalline silicon is very small, there is no limitation in a position of the particles. Therefore, when the boundary defect is used as the recombination center, the position of the grain boundary is an optional one within a whole element.
Many boundaries are within and in the vicinity of the high concentration layer. As a result, at the grain boundary, impurities are segragated, which adversely effects the element property. Such phenomena will now be practically described.
FIG. 1 shows the current to voltage characteristics of a PN junction diode. In the diagram, a solid curve indicates the ideal preferable characteristics. However, actually, since a dark current component is caused due to the electric field breakdown, the characteristics become as shown by a broken line.